Java-based call animations which cover the levels from Mainstream
to C3A. Also includes a number of square games, so you can see (for
example) how the C1 call "In
your Blocks, Square Thru" works when dancing hex (right- or
control-click on a dancer to enable hexagon squares).

Bill Ackerman's C4 Calls, Concepts, and Notions

A useful book for high challenge dancers by a long-time Tech
Squares member. Part 1 covers
calls, part 2 covers
concepts, and part 3 covers
"notions"
(everything else!). Although Bill is responsible for introducing
hundreds of new dancers to square dancing, this is definitely not for
the faint-hearted!